Did the H-bomb explode? All we saw was a white flash.
Unless it was a red herring, why would the writers show Marvin Candle sustaining a hand-losing injury during the Incident if he was subsequently incinerated by the blast? (He was seen to have a prosthetic hand in the post-Incident training films.) [Edit: his hand may only have been injured.] It seems likely, then, that the plan to change the past failed and that the H-bomb did not destroy all life on the island.
My guess is that Juliet precipitated the Incident when she smashed the warhead but that the "energy pocket" instantly neutralized the nuclear blast (although something levelled the statue). [Correction: the statue was destroyed before the Incident.]
In addition, the Incident may well propel certain characters (perhaps those who were physically touched by Jacob) from 1977 to 2007. This would allow for the Dharma Initiative to continue its work and would account for the disappearance of the non-Dharma people by the time of the 1992 Purge. Plus, I think Dharma may have fulfilled its plot utility by this point. This would nicely set up the next season because everybody would be assembled on the island in 2007 to confront Non-Locke. I'll bet the opening shot will be Jack's eye opening as he wakes up from the jump to 2007 (though making predictions about Lost is usually a losing endeavour.)
It seems clear that Faraday was correct that the past cannot be changed, despite his later and tragic about-face. There have been several examples on the show where attempts to alter fate actually brought it about, a not-uncommon literary theme (e.g., Macbeth, Oedipus Rex, and the appointment at Samarra).